Local officials were alerted at 9:30 a.m. to a call of "suspicious activity" at the UniSource Energy Services Valencia Plant. An explosion had ruptured a diesel storage tank and caused what Nogales police Lt. Carlos Jimenez described as a relatively small spill that was confined to the immediate area.

Officials closed off the power plant and an adjacent car dealership. The the FBI, the federal Bureau of Alcohol, Tobacco, Firearms and Explosives and the Arizona Department of Public Services were called.

At 5 p.m. Wednesday agents were still processing the scene.

"The reason for the high-scale response is the plant is an electrical substation and critical to the area," Jimenez said, explaining that as many as 30,000 customers in the area -- the entire town of Nogales and its environs -- depends on the plant for power.

"The whole city of Nogales could have been compromised," he added.

There were no reports of injuries and authorities said they knew of no suspects or witnesses.

They described the explosive as "a suspicious device," but would not elaborate. The fuel did not ignite, Jimenez said.